Further study of factors affecting pathogenicity and further enquiry into pathogenicity mechanisms in trichomonads and trypanosomatids will be conducted by cytologic and cytochemical (light and electron microscopy), immunologic, and biochemical methods. Laboratory animals (mice, rats, hamsters, rabbits) as well as vertebrate and invertebrate (tsetse fly) cell and organ cultures will be employed in the investigations of various facets of the problem. The mechanisms of the DNA- and/or DNA plus RNA-dependent, heritable pathogenicity transformations will be analyzed in a search for the factors that may control infectivity and virulence on the molecular level. Antigenic constitution-pathogenicity relationships of the parasites will be studied by various immunologic methods. The sequence of appearance of the variant antigens in the T. brucei-subgroup trypanosomes will be investigated and the structurally identical midgut (in tsetses) and culture trypomastigotes will be compared by the quantitative fluorescent antibody methods. Biochemical changes that may be associated with adaptations of Leishmania spp. and pleomorphic T. brucei-subgroup strains to in vitro growth at 37 degrees C will be analyzed, as will be also the effect of these adaptations on pathogenicity of the trypanosomatids for laboratory animals. An attempt will be made to correlate morphologic and biochemical bloodstream-to-culture form transformations of pleomorphic T. brucei-subgroup strains, on the one hand, and pathogenicity of these parasites for mice and rats, on the other. Comparative studies of fine structure of trichomonads will be continued.